The overall objective is to obtain as detailed and extensive data as possible on the chromosome mutation frequency induced in newborns and infants by the diagnostic x-ray procedures necessary for the medical management of cardiac problems. The project is designed to obtain information on three main points: (1) the relation in vivo of the frequency of chromosome aberrations to radiation dose in the range used for pediatric radiology to determine whether a linear dose-response relation exists at the low exposure levels (1 to 10 R); (2) the length of time after radiation exposure during which increased frequencies of aberrations can be demonstrated, the follow-up period to cover at least five years and to include information on any changes in the physical condition of the subjects; and (3) the "spontaneous" (cause unknown) chromosome aberration frequency in normal children from birth to age 5 years to serve as a baseline for the data on the irradiated children and to determine whether the frequency varies as a function of age in the absence of radiation.